1. Technical Field
Embodiments of the present invention are related to the field of chemical-mechanical polishing, and in particular, to conditioning pads for chemical-mechanical polishing.
2. Description of Related Art
Chemical-mechanical polishing (“CMP”) is a commonly used technique for planarizing a film on a semiconductor wafer prior to processing of the wafer. CMP often requires an introduction of a polishing slurry onto a surface of the film as the wafer is being mechanically polished against a rotating polishing pad.
Use of the polishing pads, as received from the suppliers, may result in significant variations in removal rates during planarization of the wafers. Pad “break-in” is used to re-condition the surface of the pad prior to use in the manufacturing process for semiconductor wafers. For example, in some cases, the break-in procedure may remove a top impervious, hydrophobic layer. The break-in procedure consists of polishing of the dummy wafers using new pads. The exact number of the wafers to be polished to achieve the desired initiation is determined empirically, and is used indiscriminately for different pad types and lots. In general, the number of wafers used should depend on the pad type, CMP process conditions, and a layer on the dummy wafers used for break-in. In one illustrative CMP process, the pad is heated to as high as 100° C. due to mechanical friction between pad and wafer. A typical break-in process consists of the series of approximately 5 minutes long wafer polishes of up to 30 wafers.
It has been discovered that an un-intentional benefit of the thermal cycles of the extended pad break-in procedure is to normalize or initiate the pad properties. Hence, it is empirically known that pad break-in may help to stabilize pad CMP performance, which is determined by the pad properties. However, in this prior art break-in procedure, there is no accurate way to control change of the pad properties caused by the thermal cycling of the break-in process. Since the current state of art is based on empirical knowledge; break-in conditions are not be optimized for specific pads or CMP processes.
Pads may be made of polyurethane and, when received from suppliers, may have crystalline phases of polyurethane. These crystalline phases—which are randomly distributed within the pad as well as within a given pad lot or pad batch—may have an uncontrolled and unpredictable impact on pad performance and may affect pad stability. Hence, the existence of thermally unstable crystalline areas in the new, as received pad, makes a typical polyurethane pad unstable. It is known that when a new, as received, pad is exposed to a single heating ramp reaching a temperature of 200° C. or above, there is a disappearance of exothermic peaks that relate to the break up of the crystalline phase of the polyurethane.
Additionally, processing temperatures for polymer-based CMP pads, during the manufacturing process for the pads, is one of the major contributors to the pad life. Temperature treatment causes irreversible changes in thermoset polymers, such as polyurethane-based pads. Polymer hardness frequently changes in a broad temperature range so that small deviations in temperature may result in a large change of polymer hardness. Establishing accurate criteria for selection of a desired processing temperature is a challenge; pad processing at non-desired temperatures may affect pad CMP performance. It is desirable to conduct processing at the lowest possible temperatures. However, some desirable properties during processing may be only achieved at the elevated temperatures. For example, during one stage of the manufacturing process, softening of the pad allows for pad patterning.